What does diathermal mean?
Diathermal is an adjective used almost exclusively in physics and thermodynamics to describe something that permits heat to pass through it. A diathermal wall or boundary is one across which heat can flow freely, standing in contrast to an adiabatic boundary, which blocks all heat transfer. The word also applies to materials that are transparent to radiant heat: rock salt, for instance, transmits infrared radiation that ordinary glass largely absorbs, making salt a classic example of a diathermanous substance. Derived from the Greek elements dia- ('through') and therm- ('heat'), the term entered scientific English in the nineteenth century alongside related forms such as diathermic and diathermancy. Because it belongs to formal technical registers, readers rarely meet it outside textbooks and journal articles, yet within thermodynamics it performs a precise and indispensable role, marking the crucial distinction between systems that exchange heat with their environment and those that do not.
adjectivePermeable to heat or radiant heat; allowing heat to pass through. In physics, especially used of substances that transmit radiant heat rather than absorbing it.
- Of a substance or medium: permitting the passage of heat, especially radiant heat (also called diathermanous).
- Of a thermodynamic system or boundary: allowing exchange of heat with its surroundings, as opposed to adiabatic.
"Rock salt is notably diathermal, transmitting radiant heat that ordinary glass absorbs."
"In the thought experiment, the gas is sealed inside a diathermal cylinder so that heat can flow in from the surroundings."
"Physicists distinguish diathermal walls, which conduct heat between systems, from adiabatic ones, which insulate them completely."
A pane of glass blocks the sun's radiant heat, but a crystal of rock salt lets it straight through — that's what it means to be diathermal.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of diathermal
Diathermal derives from the Greek prefix dia-, meaning 'through', combined with thermos or therme, meaning 'heat' — the same root that gives us thermometer, thermostat, and thermodynamics. The compound was formed in the nineteenth century as physicists developed the vocabulary of thermodynamics, parallel to formations like adiabatic. Its close relatives include the noun diathermancy and the adjective diathermanous, both coined for the specific property of transmitting radiant heat.
Related word forms
How diathermal is actually used
Diathermal is a technical term confined largely to physics and thermodynamics; in everyday writing, phrases like 'heat-conducting' or 'allows heat through' are more natural. The near-synonym 'diathermanous' is preferred when specifically discussing transparency to radiant heat, while 'diathermic' appears frequently in European physics literature.
Easily confused with diathermal
Diathermy is a noun referring to the medical technique of heating body tissue with electric currents, while diathermal is an adjective describing materials that let heat pass through.
An adiabatic process involves no transfer of heat at all, whereas a diathermal process or boundary allows heat to pass through freely.